A singular platypus is the last known relative to have branched off from all other mammals some 150 million years back.

Natural Species Remain Consistent in Appearance Throughout Millions of Years

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2025 August 28 . 6:59 AM

2 min read

A singular platypus exists as the sole survivor from a lineage that parted ways from all other... — A singular platypus exists as the sole survivor from a lineage that parted ways from all other mammals approximately 150 million years in the past.

A singular platypus is the last known relative to have branched off from all other mammals some 150 million years back.

In a question posed by David Gohman Luke on Saturday, he asked about the oldest surviving mammal that has gone unchanged by evolution. While no explicit answer was found in the search results, the question likely referred to species known for minimal evolutionary changes over vast geological timescales, often called "living fossils."

One such group of animals exhibiting relatively few anatomical changes over millions of years are the monotremes, which include the platypus and echidna. Coelacanths, not mammals but illustrative of minimal evolutionary change, are also classic examples of this phenomenon.

The fossil lineage of monotremes can be traced back to the Early Cretaceous period, approximately 110–130 million years ago. This indicates that their evolutionary lineage has changed very little since then. On the other hand, some Pleistocene mammals show only minor evolutionary changes but have much younger fossils (within the last 2.6 million years).

The platypus, one of the few living descendants of an ancestor that diverged from all the other mammals about 150 million years ago, is a fascinating example of evolutionary stability. Its unique features, such as its leathery eggs and a lack of nipples, traits that don't exist in mammals that evolved after platypuses, add to its intrigue.

Nizar Ibrahim, a paleontologist at the University of Chicago, cautioned that 'unchanged' is a tricky word when discussing evolution. However, the platypus's genome has revealed secrets of mammal evolution, shedding light on its evolutionary history.

Skulls of platypus-like ancestors have been found dating back to the Cretaceous period (63 million to 138 million years ago). Meanwhile, the evolutionary split of opossums from other marsupials occurred about 65 million years ago.

Most species of opossum, like North America's Virginia opossum, are "dietary generalists." However, the adaptability of opossums allows them to live just about anywhere. More than 60 species of possum live throughout the Americas, Australia, New Zealand, Tasmania, and New Guinea.

A 2009 study published in the journal PLOS ONE traced the opossum lineage back to a sister group of marsupials called the peradectids, which lived at the time of dinosaur extinction in the Cretaceous-Paleogene period. Opossums are ancient, with little change over the last few tens of millions of years.

In conclusion, while no exact ranking or explicit naming of “two mammals with the fewest evolutionary shifts” is provided in the sources, the best supported understanding is that monotreme mammals such as echidnas and platypuses exhibit few evolutionary shifts, with fossils dating to about 110 million years ago. Other large mammals from the Pleistocene show only minor evolutionary changes but have much younger fossils (within the last 2.6 million years).